1. Field of the Invention
This invention pertains to lighting fixtures having a fluoroplastic lens over its light emanating opening and more specifically to such fixtures having such a permanently secured lens.
2. Description of the Prior Art
Commercial lighting fixtures each enclosing one or more bulbs or lamps usually include a large window opening through which light emanates from the bulbs or lamps. The normal material used for closing this window is clear glass. In some installations frosted or otherwise partially opaque glass is employed to soften or diffuse the light.
In a typical installation of a high intensity discharge lamp, such as a mercury vapor lamp, the window opening closed by a pane of clear glass may be approximately two feet by two feet. The entire lighting fixture may be one of a plurality mounted in a high ceiling over a laboratory, industrial process area, a warehouse or similar area.
Although plastics have been substituted for glass in many applications, such as for canopies of airplanes, contact lenses, lenses for photocells and many, many other applications, previous to the development of the lens described in U.S. Pat. No. 3,812,342, substitution had never been totally satisfactory in the high temperature, prolonged use, often abusive environment that high intensity discharge lamps are subjected to. An article by J. T. Barnes appearing in Lighting Design & Application, December 1972, is believed to reflect the current state of knowledge. The findings of Barnes is that for short-term use in a high temperature, high ultraviolet environment, there are some coated polycarbonates that might be considered reservedly acceptable. For general low temperature, use, some acrylics (such as Plexiglas) are acceptable. For high temperature use (over 105.degree. C.), there was no known substitute for glass. The sole exception was that in extreme breakage areas or hazardous locations with inside temperatures not in excess of 125.degree. C., polycarbonate may be substituted, provided a very limited service life is acceptable. There are so many characteristics besides accommodation to the above that have to come together in a single plastic, that it was a remarkable discovery that any plastic could be suitable. For example, for a clear light fixture lens application, there had to be good optical, low-backscatter properties, not just initially, but after months and even years of use. Since most plastics, and even some fluoroplastics, degrade when exposed to ultraviolet light, the selection of a plastic with acceptable optical properties was extremely difficult. Further, as noted above, the high temperatures attendant to high intensity discharge lamps is also a critical problem pertaining to the selection of a suitable material.
Moreover, unless there was a vast saving in weight, the economics of the substitution did not make any sense. The most common glass substitute for large panes is probably Plexiglas, which is rigid and commonly seven-thirty-seconds inch thick (approximately 250 mils). Because it optically degrades under high temperature conditions and for other reasons, it is not acceptable. Lexan, another sometime glass substitute in other contexts, does withstand high temperatures better then Plexiglas, but it yellows to an objectionable extent.
The primary advantages of glass as a fixture closure or lens include its low cost, its ready availability, its resistance to high operating temperatures, such as emanate from high wattage lamps, its resistance to changes in color and opaqueness, even over a prolonged period of time, and its uniform light transmittance qualities over the full range of the visible spectrum.
Glass as a closure for such a fixture has a number of disadvantages, however. First, it is breakable. Should something accidentally strike the glass or should the bulb within the fixture explode, the glass is very likely to break, causing not only an inconvenience to the persons working in the area, but also creating a hazard. So-called non-breakable tempered or safety glass is available and is used. But, even tempered glass does break. In such case, beads result, rather than jagged pieces, but even beads can be hazardous.
Further, glass is thought of as being relatively slick and therefore resistant to the build up of dust. However, as most persons can attest to who have had experience with dust build-up on glass, it really does not take very long for an appreciable amount to accumulate. The rapid accumulation of dust results in reduced illumination from the fixture and a requirement to clean the fixture. The more often someone has to clean the fixture, the more expensive is the maintenance.
The glass used in a fixture such as the one described above is also an appreciable percentage of the overall weight of the fixture. A lighter window closure would effect a reduction in manufacturing and shipping costs.
As previously mentioned, U.S. Pat. No. 3,812,342 reveals an opening or lens structure which is suitable for many types of lighting installations. However, one type of installation which is not covered by the structures there revealed is for the fixture which is entered from the rear for bulb replacement purposes. Such a fixture is prevalent in outdoor sporting installations (e.g., for lighting tennis courts, baseball diamonds, and the like) and is known as a sports light. For purposes hereof, any light fixture which is not entered through the window lens, such as in the manner illustrated in the U.S. Pat. No. 3,812,342, is referred to as a sports light, regardless of the actual use or installation of such light.
It is therefore a feature of this invention to provide an improved lighting fixture, especially a rear entry fixture, having a fluorocarbon lens that is permanently secured to the fixture housing:
It is another feature of this invention to provide an improved lighting fixture, especially a rear entry fixture, having a flurocarbon lens that overlaps the outer edge and is secured with respect to a rim requiring no special frame-clamping components.